Induction melting of metals in cold, self-lined crucibles

ABSTRACT

Metals such as titanium and zirconium are melted and formed by induction heating within a cold segmented metal crucible having a self-generating and self-renewing insulating lining.

United States Patent 11 1 Clites et al. I

11] 3,775,091 1 51- Nov. 27, 1973 INDUCTION MELTING OF METALS IN COLD,SELF-LINED CRUCIBLES Assignee:

F iled:

Inventors: Philip G. Clites, Silverton; Robert A.

Beall, Albany, both of Greg.

The United States of America as represented by the Secretary of theInterior, Washington, DC.

Feb. 27, 1969 Appl. No.: 802,805

US. Cl. 75/65, 75/10 Int. Cl. C221) 53/00, C22d 7/08 Field of Search75/10, 65, 84, 94,

References Cited UNITED STATES PATENTS DeLong 75/10 R Rummel..... 75/10R Murphy... 75/10 R Loung Daubersy.... 75/49 Goss 164/82 Jackson 75/84Induction Heating Process for Melting Titanium (Cold-Wall Crucibles,Segmented and Non- Segmented), Technical Documentary Report No. ML TDR64-209 (1964) (D.D.C. No. AD-449, 372) Primary Examiner-L. DewayneRutledge Assistant Examiner-Peter D. Rosenberg Att0mey-Ernest S. Cohen[5 7] ABSTRACT Metals such as titanium and zirconium are melted andformed by induction heating within a cold segmented metal cruciblehaving a self-generating and selfrenewing insulating lining.

11 Claims, 3 Drawing Figures PAIENTEDNUYZY ms INVENTORS PHIL/P 6. CL/TESROBERTA. BEALL Z J'T A rromvns BACKGROUND OF THE INVENTION Reactivemetals having high melting points such as titanium, zirconium and thelike, are now conventionally melted and formed using cold crucibles.Such crucibles are typically constructed of a metal having a high heatconductivity such as copper and are generally cooled by circulatingwater. Exemplary processes which have been developed on an industrialscale using water-cooled crucibles include vacuum-arc melting, HopkinsProcess or electro-slag melting and electronbeam melting. All of theseprocesses have inherentdisadvantages. Electron beam melting isexpensiveand requires the maintenance of a relatively high vacuum. Bothconsumable-electrode arc melting and Hopkins Process require fabricationof a consumable electrode and in many cases provide poor control ofmelting rates.

The advantages of a process utilizing induction heating have long beenrecognized and many attempts have been made to utilize this technique inthe melting of reactive and refractory metals. Of these many attempts,none has provided asatisfactory method formelting these metals on anindustrial scale. Attempts to melt refractory metals using inductionheating date back at least to 1926 as is shown by German Pat. No.518,499. This patent proposed the use of a water-cooled, metal cruciblehaving high heat conductivity. Copper and silver were recommended asbeing especially suitable. The crucible was constructed of segmentselectrically insulated from each other by materials such as mica inorder to prevent undue attenuation of the magnetic flux applied by aninduction coil encircling the cruciclosest to achieving the desiredresults. His system was successful on small-scale equipment but attemptsto utilize his technique in large crucibles resulted in molten metalshorting across the crucible segments causing melting of portions of thecopper crucibIeAttempts to use an oxide coating, such as berylliumoxide, on the interiorcrucible surface as was proposed inthe Germanpatent were also unsuccessful.

SUMMARY OF THE INVENTION It has now been found that the difficultiesassociated with the prior processes and apparatus may beover- DETAILEDDESCRIPTION or THE INVENTION- Exemplary embodiments of the inventionwill now be described with reference to the accompanying drawings,wherein:

FIG. 1 is a diagrammatic partial section of an apparatus for thecontinuous melting'and solidification of refractory metals and othermaterials.

FIG. 2 is a longitudinalcross-sectional view of the crucible employedinthe apparatus of FIG. 1 illustrating construction details and chargedistribution during operation of the apparatus.

FIG. 3 is a cross-section taken along line 3-3 of FIG. 2.

The apparatus illustrated in FIG. 1 depicts a me ferred embodiment forthe continuous production of an ingot within a controlled compositionatmosphere or vacuum environment. Crucible l is contained within ahousing or chamber 2 constructed in such a manner as to be gas-tight.Crucible l is preferably constructed of a metal having highheat'conductivity such as copper.

' The crucible is split longitudinally by at least one slit 3 so as toreduce the attenuation or shielding action which would be presented byan electrically continuous crucible. Also provided is a cooling jacketfor the crucible (not shown) which preferably comprises an integralinduction current to the work coil. The work coil is provided withcooling means and preferably comprises copper tubing having a coolingmedium flowing therethrough. 4

. I Disposed generally above and discharging into the open topofcrucible l are material feeders 5 and 6.

These feeders are of any conventional type capable of the controlleddischarge of granular or particulate solids within a'vacuum orcontrolled environment atmosphere. Feeder 5 controls the discharge of aslag material into the crucible while feeder 6 dischargescontrolledamounts of particulate ingot-forming material such as titanium sponge. g

Extending from the bottom of the crucible is shown a' formed ingotportion 7 attached to a control rod 8.

' within the crucible and to'continuously withdraw the come in anextremely simple manner. Use of a slag or fluxing agent in associationwith the metal charge produces a self-generating and self-renewinginsulating material between the crucible segments and provides a linerfor the interiorcrucible surface.

Hence it is an object of this invention to provide an apparatus andprocess for the inductive melting and forming of reactive metals. I

Another object of this invention is to provide a self-' generatinginsulating lining for a cooled metal crucible used in the inductionmelting of metals.

A specific object of this invention is to provide a process for theinduction melting and forming of titanium .and other high-meltingmetalsand their alloys.

ingot as it is formed. In the embodiment illustrated, the control rodpasses through the bottom of chamber 2 by means of gas seal 9.Encrusting the outer surfaces of ingot 7 is a frozen slag layer 10.Referring now to FIG. 2, there is shown a longitudirial sectional viewof the crucible. As illustrated, crucible 1 is divided into multiplesegmentsiby means of slits 3. Each crucible segment is provided with anintegral coolant channel 15 through which 'a fluid such as water iscontinuously circulated during operation. The crucible segments aremounted in electrical'isolation from each other on a non-segmented lowercrucible section 16. Electrical isolation of each crucible segment maybe conveniently accomplished by providing insulating member 17 betweeneach segmentand the lower crucible section. Attachment of the segmentsto the lower section may be'by use of insulated bolts (not shown) or anyother convenient method.The lowersection in turn is attached to bracketmember 18 for mounting within the vacuum chamber.

To begin a melting and ingot-forming operation, a starting stub 19attached to a control rod 8 is placed within the lower part of thecrucible. The control rod may comprise two annularly disposed pipes asshown for the circulation of coolant and may be attached to the startingstub in any convenient manner. An initial melting charge consisting ofthe metal to be melted together with a quantity of slag material is thenplaced on top of the starting stub within the area encircled by the workcoil 4. The starting stub is preferably of identical composition withthe material to be melted.

Power is then applied to the work coil and the initial charge is heated.As heating progresses the slag melts, running down and solidifying inthe annular area between the starting stub and the crucible wall. A thinlayer of slag 20 also freezes on the cold crucible wall thus providingboth heat and electrical insulation between the cold crucible and thehot metal charge. Continued heating melts the initial charge of metaland the top portion of the starting stub thus welding the two togetherto forma solidified ingot portion 21. A molten pool of metal 22surmounts the solidified ingot and is surrounded by slag 23.

During operation, levitating forces induced by the electrical fielddistort the molten metal pool into the general shape shown. Buoyantforces exerted by molten slag surrounding the metal probably contributeto the levitating effect. Solid metal is added to the molten pool asfast as it can be melted and assimilated by the pool. The formed ingotis withdrawn from the bottom part of the crucible at a rate such thatthe level of molten metal and slag remains constant. Slag is added,either intermittently or continuously, to make up that carried out ofthe heating zone as a coating on the ingot.

Turning now to FIG. 3, there is shown a crosssectional view of thecrucible taken generally along the line 3--3 of FIG. 2. The crucible 1is divided into four equal segments by means of longitudinal slits 3.Each crucible segment is made up of an inner wall member 31 and an outerwall member 32 which together with spacing members 33 define a coolantchannel 15. In a preferred embodiment, spacing members 33 and outer wallmember 32 are offset circumferentially relative tothe edge of inner wallmember 31 to form a flanged recess as is shown in the drawing. Withinthe longitudinal recess may be placed an insulating spacer member 35which may g comprise a tubular or rod-like ceramic member. An aluminathermocouple insulator was found to be completely satisfactory for thepurpose. Spacer member 35 need not seal slit 3 but acts to keep thesegments in physical alignment and provides a physical barrier to theunimpeded flow of molten slag through the slits before the slag has hadtime to freeze.

Positioned within the crucible is a solid ingot 21 formed during themelting operation. Completely surrounding the ingot and separating itfrom physical or electrical contact with the crucible is a solidifiedslag layer 20. This slag layer extends within slits 3 thus aiding themaintenance of electrical isolation between ad jacent segments.

The process and apparatus may be used forthe melting of a wide varietyof materials including the common commercial metals when economicconsiderations justify its use. The process and apparatus, however, isparticularly applicable to the melting and forming of tita- 4 nium,zirconium, hafnium and alloys of those with other metals.

Fluxes or slags suitable for usein the process include anymaterialswhich fulfill the following requirements: (1) The slag must be anelectrical insulator in its solid form. (2) It should have a fairly highmelting point but its melting point must be significantly lower thanthat of the metal being processed. (3) It must be nonreactive toward andinsoluble in the molten metal being processed. (4) The slag must be atleast relatively sta' ble atthe temperatures used in the process, and(15) it desirably has a low volatility at process temperatures.Obviously choice of a suitable slag is highly dependent upon the metalbeing processed. The alkali and alkaline earth metal fluorides, andespecially calcium fluoride, have been found to be excellent slagmaterials when melting such metals as titanium, zirconium and theiralloys. i

It has been found that any physical form of the metal which is capableof being introduced into the melting crucible at a controlled rate isamenable to treatment by the process. Physical formsof metalsuccessfully melted and formed by'the process include cast rod stock,pressed and compacted sponge rods, metallic sponge such as thatproducedby the Kroll'process and particulate scrap of varying size and shape.Slag requirements vary depending'upon the crucible size, slagcomposition and metal being processed. Generally slag requirements willrange from about 2 to about 20 percent by weight of the metal processed.Some of the slag may be recovered and reused in the process.

It is necessary that the melting and forming of reactive metals'such astitanium be performed either under vacuum or in an inert gas atmosphere.Inert gases such as argon or heliumare satisfactory as a protectiveatmosphere. While process pressure may range fromhigh vacuum tosuperatmospheric, it is generally preferred to operate the process atsubatmospheric pressures. In

the melting of titanium using a calcium fluoride slag, for example, itwas found that operation at about :6 atm using helium as theinert gasprovided satisfactory operation and substantially reduced slagvolatilization,

EXAMPLE 1 5 A series of preliminary tests were performed to investigatethe action of fluxing materials during the heating of a metal charge ina cold crucible.'A water-cooled split copper crucible having a singlelongitudinal slit was constructed. A cylinder of mild steel was placedwithin the crucible together with'a charge of solid calcium fluoride.The steel i'cylinder had a diameter slightly less than the insidediameter of the crucible. Inductive heating of the cylinder was thencommenced.

As the steel heated, calcium fluoride incontact with the cylinder wasmeltedrwith continued heating, the

entirecharge of calcium fluoride melted and formed a cover of moltenslag. As had been theorized, a thin layer of slag solidified against thecold wall of the crucible thus completely insulating the crucible fromthe heated metal.

All tests were conducted in an open environment XAMP E l A crucible,such as that illustrated in FIG. 2, was constructed. This crucible had 4longitudinal slits dividing the crucible body into 4 segments. Insidediameter of the crucible was 3% inches and length of the segmentedsection was 8-% inches. Thecrucible segments were bolted to anon-segmented lower section also having an inside diameter of 3% inches.Alumina thermocouple insulators were used as spacing membersbetweensegmentsgEach crucible segment was insulated from the lower section bymeans of a'Micarta ring.

A work coil consisting of 7 turns of l inch heavywalled copper tubingwas placed coaxially around the upper portion of thecrucible. in thisway, melting occurred in the upper part of thecrucible while cooling andsolidification took place in the lower part. Power for inductivelyheating and melting was supplied by a 10 kc motor-generatorrated at 7 5kw. The entire crucible assembly was then installed within a vacuumchamber. A single-stage blower connected in series with a mechanicalpump was used to evacuate the chamber. Side feeding units were, alsoinstalled for adding slag and metal to the crucible..

A general procedure for melting metals was established as follows:First, a starting stub consisting of a cylinder of the metal to'bemelted is placed within the lower portion of the crucible. Diameter ofthe stub must be slightly less than the insidediameter of the cruciblein order to avoid shorting out thecrucible segments. The top of the stubis located generally at the bottom of the work coil. Next, an initialcharge of the metal to be melted together with a quantity of slag isplaced within the crucibleon top of the stub.

Power is then applied to the work coil resulting in the heating of themetal charge. As-heating proceeds, the slag is first melted forming asolid insulating lining on the interior of the crucible and within thelongitudinal slits. Heating is continued to form a molten pool of metalessentially covered and surrounded by molten slag. As soon as a mdltenpool of metal is established, feeding of metal into the pool iscommenced. As material is added to the 001, the resulting ingot iswithdrawn through the b ttom of the crucible.

EXAMPLE 3 crucible and until the original metal charge had formed amolten pool. Power level was then increased to 45 to 50 kw and feedingof titanium sponge was commenced. Sponge was dropped into the moltenpool from the outlet of a side feeder located ll inches above the top ofa the crucible. Rate of sponge addition was limited by the rate at whichsponge was melted and assimilated by the molten pool. Make-up slag wasadded as the slag cover was depleted by solidification of a slag layeron the outer surface of the ingot.

As metal was added, the ingot was withdrawn to maintain a constant levelof the ingot within the pool.

Feeding of sponge'continued until the supply within the side feeder wasexhausted. Themelting rate was 0.44

pounds per minute and power consumption was l-.7.

kwh per pound. At the conclusion of the run, power to the work coil wasterminated, the ingot was allowed to cool and was then removed from thecrucible. The cooled ingot had a continuous calcium fluoride coating onthe order of l/ 16inch ingot was good.

Observations during the run indicated that levitating forces acting onthe molten metal coupledwith buoyancy exerted by the molten slag causeda part of the molten metal pool to be lifted above the surface of theslag. Field strength was most intense at the crucible slits as was shownby the factthat metal adjacent to the slits was forced inward. Whenviewed from above, molten metal protruding above the molten slag had theshape of a four-pointed star; the points being located midway betweenslits. Since molten calcium fluoride is a good electrical conductor,there was some question as to whether it was heated by inducedcurrentflow within the slag. Absence of any visible levitation of theslag indicated that at least most of the slag heating was by conductionfrom the metal.

E MPLE .uum distilled titanium sponge, leached'a'n'd dried titaniumspongewas used as the initial charge and feed material. It had beenexpected that this sponge would be more difficult to melt than thevacuum distilled sponge.

Using the apparatus and general procedure set out in Example 2, atitanium metal charge was melted. The

starting stub was a 5-inch length of titanium rod havinga diameter of3-% inches. Attached to the stub was a water-cooled control rodextending through the bottom of the vacuum chamber. ;The control rod wasused to adjust the level of the ingot in the crucible and to withdrawthe ingot as it formed. An initial charge consisting No difficultieswere encountered although'the melting rate was somewhat lower than thatachieved in Example 3 and the sidewalls of the produced ingot wererougher. Overall melting rate was 0.32 pounds per minute at a powerconsumption of 2.3 kwh per pound.

EXAMPLE The procedure of Example 2 was repeated using yet another typeof feed material in order to determine the utility of the process-forreclaiming scrap. Solid titanium parts including broken tensilespecimens, chopped sheet and sections of ingots were used as theoriginal crucible charge. No difficulties were experienced in meltingthe material. There were difficulties in feeding the scrap material,however, due to the fact that the side feeder was not well adapted todispensing the varied sizes and shapes of the scrap. Pieces of scrapweighing as much as 15 g were dropped into the molten pool from a heightof 1 1 inches with virtually no splashing of metal. The resulting ingotwas of excellent quality.

thick.'Surface quality of the I EXAMPLE 6 Additional runs were madeusing consumable titanium feed rods as the starting charge. Feedmaterial was in the form of pressed sponge compacts and swaged rodsofpreviously melted titanium. The feed rod was introduced from the top ofthe crucible and was fed downwardly until its lower end was submerged inthe slag. As metal at the end of the feed rod melted and transferred tothe pool of molten metal in the crucible, the feed rodwas progressivelylowered while the ingot was withdrawn from the pool. Quality of theresulting ingots were excellent.

EXAMPLE 7 One attractive application of this process would be toinduction melt titanium sponge to form an ingot for use as a consumableelectrode for vacuum-arc remelting into a final ingot. Induction meltingand forming would eliminate the pressing and welding operations requiredto produce a first-melt electrode for vacuum-arc melting and would alsoconstitute the initial melting step.

Induction-melted ingots prepared from all varieties of startingmaterials illustrated in the previous exam-' ples were found to besatisfactory as electrode stock. No machining of the outer electrodesurface was necessary. Due only to the limited capacity of theexperimental furnace, it wasnecessary to weld several ingots together toform an electrode of suitable length for vacuum-arc remelting.

Ingots formed by vacuum-arc remelting of inductionmelted electrodes wereof excellent quality. Impurity content of such ingots were compared tothe impurity content of a standard vacuum-arc remelt of the same lot oftitanium sponge. lngots melted by induction melting contained a slightfluorine contamination and ingots from sponge with a high hydrogencontent were I higher in hydrogen than standard vacuum arc meltedingots. Other impurities in induction-melted ingots were at a levelcomparable to vacuum arc melted ingots.

EXAM RLEE? A run was attempted without the use of slag. The apparatusand techniques used were otherwise unchanged from those employed in thepreceding examples. For i against the sides of the crucible causing animmediate decrease in the heating rate. It was thereafter impossible tomaintain a full pool of molten metal. Titanium sponge added to thecrucible by side feeding did not completely melt.

The crucible was then allowed to cool and the ingot was removed. Uponinspection of the crucible, evidence of arcing between adjacent segmentswas noted. Melted metal had run into the crucible slits causing arcingbetween adjacent segments and resultant damage to the crucible walls. vi

The crucibles used in the experimental work were of circularcross-section and ingots produced by the process conformed to thatshape. Other crucible shapes may be used, however, to produce ingots ofdifferent cross-section.

As has been demonstrated in the examples, this process has capabilitiesnot present in any other known method of melting and forming ingots.Specifically, no other process is capable of handling the variety ofphysical forms of feed metal to produce ingots on acontinuous orsemi-continuous basis.

What is claimed is:

l. A coreless induction furnace for the melting and forming of metalswhich comprises a hollow; elongated, electrically isolated metalcrucible open at the top and bottom and of uniform cross-sectionthroughout the length thereof and having crucible sidewalls divided bylongitudinal slits into at least two segments, each segment beingmaintained in electrical isolation from every other segment, means forcooling'the metal crucible, means for inducing an alternating electriccurrent within a metal charge contained in the crucible and means forcontinuously generating and maintaining an insulating lining on theinner surface of the crucible and within the longitudinally extendingslit said insulating lining comprising a metallic compound havingelectrical insulating properties when solid and having a I melting pointbelow that of the metal to be melted and formed within the crucible.

2. The apparatus of claim 1 wherein the insulating lining comprises analkaline earth metal fluoride.

3. A process for melting andforming metals which comprises:

a. placing an initial solid charge of a metal chosen from the groupconsisting of iron and nickel base alloys, titanium, zirconium, hafniumand their alloys, together with a slagging material, said materialcomprising a metallic compound having electrical insulating propertieswhen solid and having a melting point below that of the metal charge,within a metallic crucible open at the top and bottom and. of uniformcross-section throughout the,

d. continuously cooling the crucible to form and maintain a frozen slaglining on the interior surfaces of the crucible and filling thelongitudinal slit, and e. adding additional solid metal to the moltenpool to be melted and assimilated therein. 4. The process of claim 3wherein the pool of molten slag and metal within'the crucible isestablished on top of a metallic starting stub positioned within thelower portion of the crucible and wherein the starting stub is withdrawnat a rate substantially equal to the rate of addition of solid metal tothe molten pool to continuously form an ingot conforming to thecross-sectional shape of the crucible.

5. The process of claim 4 wherein the cross-'sectiona shape of thecrucible is circular. i

6. The process of claim 4 wherein the initial metal charge andadditional solid metal is chosen from the group consisting of titanium,zirconium, hafnium, and their alloys.

7. The process of claim 6 wherein the metal is titanium and wherein theslagging material comprises an alkaline earth metal fluoride.

8. The process of claim 7 wherein the slagging material comprisescalcium fluoride.

environment comprises an inert gas atmosphere.

* wk k l

2. The apparatus of claim 1 wherein the insulating lining comprises analkaline earth metal fluoride.
 3. A process for melting and formingmetals which comprises: a. placing an initial solid charge of a metalchosen from the group consisting of iron and nickel base alloys,titanium, zirconium, hafnium and their alloys, together with a slaggingmaterial, said material comprising a metallic compound having electricalinsulating properties when solid and having a melting point below thatof the metal charge, within a metallic crucible open at the top andbottom and of uniform cross-section throughout the length thereof andhaving sidewalls divided by longitudinal slits into at least twosegments, each segment being maintained in electrical isolation fromevery other segment; b. establishing a pool of molten slag and metalwithin the crucible by applying an alternating electrical flux suppliedby a primary induction coil surrounding an upper portion of thecrucible; c. maintaining the temperature within a lower portion of thecrucible at a temperature substantially less than that of the upperportion; d. continuously cooling the crucible to form and maintain afrozen slag lining on the interior surfaces of the crucible and fillingthe longitudinal slit, and e. adding additional solid metal to themolten pool to be melted and assimilated therein.
 4. The process ofclaim 3 wherein the pool of molten slag and metal within the crucible isestablished on top of a metallic starting stub positioned within thelower portion of the crucible and wherein the starting stub is withdrawnat a rate substantially equal to the rate of addition of solid metal tothe molten pool to continuously form an ingot conforming to thecross-sectional shape of the crucible.
 5. The process of claim 4 whereinthe cross-sectional shape of the crucible is circular.
 6. The process ofclaim 4 wherein the initial metal charge and additional solid metal ischosen from the group consisting of titanium, zirconium, hafnium, andtheir alloys.
 7. The process of claim 6 wherein the metal is titaniumand wherein the Slagging material comprises an alkaline earth metalfluoride.
 8. The process of claim 7 wherein the slagging materialcomprises calcium fluoride.
 9. The process of claim 6 wherein themelting and forming of a refractory metal is performed in a non-reactiveenvironment.
 10. The process of claim 9 wherein the non-reactiveenvironment comprises a vacuum.
 11. The process of claim 9 wherein thenon-reactive environment comprises an inert gas atmosphere.